Can I vote none of the above?
The Buffy/Angel relationship was a little tortured and dark. Too angst ridden for my tastes. The pining and the "we're in love but can't really be together" bit just started to wear on me.
The Buffy/Spike relationship was a wee bit abusive. Too sadomasochistic for my tastes. I mean, Spike was stalking Buffy, he even tried to rape her. He may have a soul now, but the only way that could excuse his actions is if it makes him a different man. If it does that, then for all intents and purposes Buffy and Spike just met (a little soon for true love wouldn't you say?)
The Buffy/Riley relationship was more to my liking. It had warmth and compassion and was actually capable of going somewhere. Too bad Riley totally sucked. If I didn't hate the character so much I'd be in favour of them, but I hate him so I'm glad this relationship is over too.
The only relationships I've ever really liked on the show were Giles and Jenny, Xander and Anya, and Willow and Oz. Willow and Tara might have had a shot at me liking them together... but Tara, unfortuneately, was no Oz. Seth Green just comes off so well on screen, he's a tough act to follow. Doesn't that just suck, all my favorite relationships are over.
P.S. - Die Kennedy! Die! Tara was no Oz, and Kennedy is no Tara!